May22 M1-Embryonic development of vertebral column and limbs 1 Flashcards
when vertebra form
- after nervous system forms (will associate with it)
- early in gastrulation when notochord forms
name of the earliest event for body innervation and segmentation
principle of original innervation
principle of original innervation
- in gastrulation, segmentation of body because newly-formed mesenchyme makes a segment pattern
- a midline segmented vertebral column + paired nerves form. is the basis for tissue innervation
2 key things that happen in gastrulation
- formation of all mesenchymal stem cells
2. initial hox gene expression
first structure forming in gastrulation
notochord in midline
what forms the notochord
intra-embryonic mesenchyme
3 functions of notochord
- induce overlying primitive ectoderm to form neural plate (neurectoderm of the future neural tube) (prechord also called prechordial plate also helps for this induction)
- induce the PNS
- induce the vertebral column later on
portions of the neural plate (formed by primitive ectoderm after notochord induced that)
- forebrain, midbrain, hindbrain
- spinal cord crossing all that in the middle
cells on the periphery of the neural plate after it forms and their function
NCCs
-will form the PNS later on
how neural plate becomes neural tube (with name of process)
- neurulation*
- neural plate forms and forms a neural tube
- surface ectoderm folds and pinches, forming the skin. comes on both sides and closes over the neural tube
how NCCs on periphery of neural plate behave during neurulation
- initially, have neural plate + NCC on their periphery + surface ectoderm around all that
- NCCs move towards the axis of folding of neural plate (as if guiding the folding)
NCC type of cells
stem cells
(imp) what NCCs do after neurulation
- migrate into whole embryo
- will induce the PNS most importantly like segmental dorsal root ganglia (among other tissues)
one consequence of neural tube formation
induces the formation of mesenchyme
during gastrulation, what cells (and in what region) induce the neural plate
- have pritimive streak and node (node = most mesenchyme forming there)
- mesenchyme goes up in the middle axis to form neural plate by inducing notochord
during gastrulation, how is segmental pattern established
some strike and node mesenchymal cells migrate laterally to form segmental pattern
first transcription initiation events in the embryo
hox genes (are segmental genes so allows segmental pattern) expressed by mesenchymal cells of strike and node that went laterally in gastrulation
how hox genes spread
- hox gene expressing region becomes larger, in the cranio-caudal direction
- initially loose mesenchyme and then aggregates (forming somites)
from near the neural plate (with underlying notochord) to outside of the trilaminar embryo, names of layers of (mesoderm) segmentation
- paraxial mesoderm (closest to axis of neural plate) (somites)
- intermediate mesoderm
- lateral plate mesoderm
key cellular event for gastrulation and NCC formation and key cellular event for mesenchyme aggregation
- epithelial mesenchymal transition (EMT): for gastrulation and NCC formation
- MET for mesenchymal aggregation (after it becomes epithelial in nature)
name of the structures formed by mesenchyme aggregation and what they express
somites (paraxial mesoderm)
- express retinoic acid which is an important regulator of the hox gene
- express hox gene
consequence of retinoic acid expression by somites
- induction of the adjacent spinal cord
- FUNCTIONAL (but not structural) separation of this spinal cord (in segments)
when do neurulation and somite development happen with respect to each other
both together at the same time at 22-25 days
somites are what region of mesoderm on the side of the neural plate (segments formed by hox genes)
paraxial mesoderm
last region to close (end of neural tube formation) in neurulation
caudal region
important concept that links spinal cord and somites (mesenchyme aggregation (MET) and retinoic acid, etc.)
molecular linking between the spinal cord and the somites (the somites will lead to the spinal cord via retinoic acid)
after neurulation, what do NCCs form along the neural tube (think of what they become)
- aggregate to form the dorsal root ganglia that are segmented (associate with a somite and a region of the spinal cord)
- will also form the meninges around the spinal cord and brain
what’s a dorsal root ganglion in the spinal cord
- cell bodies with dendrites projecting to tissues (and periphery, rest of the body) and axons projecting to the spine
- brings sensory info to the spine
segmentation of the spine with somites and dorsal root ganglia: what is the clinical relevance
explains the dermatomes and the myotomes patterns
what forms the meninges (dura mater, arachnoid, pia mater)
NCCs
what forms the SS chain and the PNS
NCCs
after you have the neural tube completely formed with dorsal root ganglia, meninges, etc. what do somites do
on each side of the neural tube (of every neural tube level), somites form a cavity with a dermo-myotome on its outside and a sclerotome on its inside
how do somites form cavities on each side of the neural tube
because mesenchymal stem cells in somites become differentiated, commit to form diff cell types (dermo-myotome cells, sclerotome cells)
dermo-myotome will form what
- the dermis under the epithelium
- the myotome will form the skeletal muscle (stem cells will differentiate)
what will the sclerotome form
CT, specialized CT, cartilage and bone
as the neurulation is completed, what does the lateral plate mesoderm do
it forms a cavity (the intra-embryonic coelum) separating the somatopleure (body wall) from the splanchnopleure (gut wall)
what will the sclerotome do relating to the neural tube and what induces that
- differentiates and migrates around the neural tube to form the vertebra
- NCCs induce that
gene that the notochord and floor plate of the neural tube continue to form (and have been forming since before neurulation)
sonic hedgehog. (SHH). It diffuses through neural tube, towards the sclerotome and this expression of SHH keeps diffusing laterally
other gene that diffuses towards the sclerotomes
bone morphogenetic protein 4 (BMP4)
-comes from roof plate of the embryo
what is the effect of SHH FROM THE NOTOCHORD***on sclerotomes
- induces EMT in ventro-medial somitic cells (initially mesenchymal SCs did MET to aggregate and form somites)
- induces these new mesenchymal stem cells to make Pax-1
- Pax-1 is important for cartilage and bone production by the sclerotome
function of Pax-1 expressed by sclerotomal cells
- is a TF
- important for cartilage and bone production
effect of BMP4 on the sclerotomes
induces sclerotome mesenchymal stem cells expressing Pax-1 to migrate around the neural tube
pattern of migration of sclerotomes around the neural tube
go towards the roof to form the lamina and go outwards to form the transverse processes of the vertebrae
what determines the shape of the vertebra
diff types of hox genes being expressed at this stage still (where vertebrae being formed by sclerotome wrapping around) and will determine this segmentation: cervical, thoracic and lumbar vertebrae
structures formed by sclerotome migrating around notochord and neural tube on both sides
- lamina (fuses to form spine of the vertebra)
- centrum (body of the vertebra) and IV disc
- transverse process
- pedicle
what induces the lamina to make it fuse and form the spine of the vertebra
the roof plate
2 sources of formation of the vertebra and clinical relevance
ventral and dorsal aspect
-bad closure of neural tube = dorsal portion poorly stimulated and not formed properly
what determines the cartilage model of vertebrae and what gives each vertebra its unique structure (needed for its spinal level)
cranial-caudal expression of a unique combination of hox genes. important until chondrification (basic form of vertebra being established) begins
what happens if the neural tube/the lamina don’t close right
get spina bifida (a bifid vertebral spine (spinous process))
possible defect in the cervical spine if there is an abnormal hox code expression and significance clinically
cervical rib (formed by transverse process) -can compress the brachial plexus and the nerves going to rib 1
what can be the cause of an abnormal hox code expression leading to a cervical rib
retinoic acid abnormalities during the termination of the hox cord (end of hox cord pattern being established)
what forms the centrum and IV disc at one spinal level
2 sclerotomes (one from each side)
what does the notochord do as the vertebra is being formed by the 2 sclerotomes
- regresses
- helps for IV disc formation
- will always remain as the nucleus pulposus in the IV disc
how many cervical vertebra and how many cervical nerves
7 C vertebra
8 C nerves
shift between vertebra level and nerves level
position of nerves relative to vertebrae in cervical and thoracic spinal levels
C: nerve above corresponding vertebra T: nerve is below corresponding vertebra C7 nerve is above C7 vertebra C8 nerve is below C7 vertebra T1 nerve is below T1 vertebra
shape of the vertebral column in the embryo
C shape (kyphosis all the way) inwards
step after C shape column in the embryo
sacrum starts forming
step after sacrum starts forming in the C shape column in embryo
neck starts forming
step after neck and sacrum formed in the C shape column
get normal spine with C lordosis, T kyphosis, L lordosis, S kyphosis
(important) what happens to the spine with age
IV discs contribute to the length of the whole vertebral column. because IV discs regress with age, the vertebral column becomes shorter and and its curvature changes
